1. Field of the Invention
This invention concerns a system and methods usable to dry liquid ink images.
2. Description of Related Art
Inkjet printing quality, such as, for example, uniformity of ink density, contrast of the ink with respect to the paper on which the ink is placed, lack of smearing, etc., is generally highly dependent upon the quality of the recording medium on which the ink is placed and the surface tension of the ink. Inkjet printers that use high surface tension, and therefore slow penetrating inks, including water soluble inks, require that care be taken to avoid smudging of the ink on the recording medium too soon after the ink is printed and to avoid offset problems, i.e., transfer of the ink onto an object that comes in contact with the recording medium, such as, for example, another piece of paper or a human hand. In general, it is desirable to be able to actively dry an inkjet printed image sufficiently so that the image bearing recording medium may be contacted by another object without smudging within 3 seconds after the image has been printed. Drying the printed ink is often accomplished naturally, i.e., by ambient air drying, but active drying is also used, e.g., drying with a source of heat such as, for example, a radiant heater, a microwave heater, or hot gas.
For example, U.S. Pat. No. 4,340,893 to Ort describes a scanning inkjet printer with an ink drying apparatus on the carriage where the drying apparatus includes a dryer body 40 that directs unheated or heated air onto the printed ink, and even provides for re-circulation of the air. The humidity of the air may be monitored to obtain an indication of the drying capability of the system.
U.S. Pat. No. 4,970,528 to Beaufort et al. discloses a uniform heat flux dryer system and method for an inkjet printer using an infrared bulb.
U.S. Pat. No. 5,349,905 to Taylor et al. teaches using a microwave dryer to dry a thermal inkjet printed image.
U.S. Pat. No. 5,502,475 to Kaburagi et al. teaches using an electrical resistance heater with a temperature control unit to dry an inkjet printed image.
U.S. Pat. No. 5,631,685 to Gooray et al. teaches a microwave dryer for an inkjet printer.
U.S. Pat. Nos. 5,713,138, 5,901,462 and 5,953,833 to Rudd teach a dryer for wet coatings, including printing inks, using re-circulated heated and pressurized air which impinges on the wet coated recording medium, and the use of energy emitters such as radiant heating elements.
U.S. Pat. No. 4,566,014 to Paranjpe et al. discloses a method of sheet feeding to enhance dryer operation and which discloses different types of dryers for ink drops on sheets, including a radio frequency dryer and a drying system employing dried and heated air blown at high velocity onto a sheet of paper to accelerate drying of the ink deposited on the sheet of paper.
U.S. Pat. No. 5,214,442 to Roller discloses an adaptive dryer which varies the feed rate of inkjet printed pages through a dryer and the temperature of the dryer, and discloses a microwave dryer and a convective dryer.
U.S. Pat. No. 5,140,377 to Lewis et al. discloses a xerographic printing apparatus in which toner material is thermally fused and fixed onto a surface of a copy sheet by condensing water vapor on the surface of a copy sheet.
The two-phase drying systems and methods according to this invention improve the quality of liquid ink printing with high surface tension/slow penetrating inks, such as water soluble inks, on different media, including paper.
This invention provides systems and methods for rapidly drying liquid ink that use an active two-phase drying system. This invention separately provides systems and methods for actively drying liquid ink that use a brief water condensation interval to heat the liquid ink and recording medium.
This invention further provides systems and methods for actively drying liquid ink that follow the water condensation interval with a period of relatively low velocity laminar air flow.
This invention next separately provides systems and methods for actively drying liquid ink following the laminar air flow drying that use a short period of modulated re-circulating hot air flows impinging on the wet ink. This results in the ink being dried in a rapid continuous manner equal to the printing rate so that no subsequent drying period is needed.
Various exemplary embodiments of a system according to this invention include a dryer which has a moist air circulating system. In various exemplary embodiments, the moist air first comes into contact with the liquid ink in a relatively low pressure laminar flow region according to this invention. In various exemplary embodiments, the liquid ink is then placed in a second higher pressure turbulent region in which the moist re-circulated heated air is driven against the liquid ink. In various exemplary embodiments, the moist heated air is driven by a fan and contains air impingement plates, one for an air heater plate and one to direct heated air against the liquid ink. In various exemplary embodiments, a thermally insulating enclosure is provided for the dryer. The impingement plates have openings, including holes and/or slits arranged in some pattern, that provide desired air flow conditions. The air impingement plates increase the velocity of the moist air pattern with respect to air being blown by an air re-circulatory element, such as, for example, a fan over the velocity that would occur without the air impingement plates. This increased air velocity increases the heat transfer coefficient in the dryer, improves the efficiency of the heating of the air in the dryer by the heater plate, and speeds up removal of moisture from the recording medium on which the liquid ink has been printed.
In various exemplary embodiments of the systems and methods according to this invention, drying the liquid ink occurs in two stages. The dew point, or condensation temperature, of the moist air circulating in the dryer is built up to, and maintained at, a value which is well above the temperature of the incoming recording medium and the liquid ink. As a result, for a short period of time, until the temperatures of the recording medium and liquid ink increase in the dryer, a small amount of moisture is condensed onto the surface of the incoming ink and the recording medium on which the ink is deposited. The heat liberated during moisture condensation heats up the liquid ink and the recording medium on which it is deposited to the dew point temperature of the re-circulated drying air. This results in restricting further moisture condensation and allowing the drying process to begin within the laminar flow and impingement flow air drying sections. As a result of the initial moisture condensation, the ink image is uniformly heated and its viscosity and surface tension are lowered, whereby the quality of the printing or imaging is improved in many aspects, including the uniformity of ink distribution and ink density, and contrast between the ink and the recording medium. In the laminar flow and impingement flow air drying sections, the liquid ink is rapidly dried.
In various exemplary embodiments of the systems and methods of this invention, the distance between the heating element and its impingement plate remains constant. The recording medium and its air flow plate also remains substantially constant as the recording medium moves through the laminar flow dryer. In various other exemplary embodiments of this invention, in the turbulent flow region, the spacing between the air impingement plate and the recording medium can either remain constant or it can vary as the recording medium moves through the dryer. In various exemplary embodiments, when the spacing changes, the spacing or gap between that air impingement plate and the recording medium is greatest at the entrance to the turbulent impingement plate dryer and is smallest at the exit from the turbulent dryer. In various exemplary embodiments of the systems and methods according to this invention, the moist heated air is brought into contact with the back side of the recording medium, (i.e., the side of the recording medium on which the liquid ink is not deposited) employing an impingement flow plate to produce turbulent flow and also the heated moist air is brought into contact with the front side of the recording medium employing a flow plate to produce laminar air flow.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.